The intellectual abilities of England's brightest teenagers have declined rapidly in three decades, according to a study which suggests schools are failing to stretch the brightest pupils in their drive to improve education for the majority of children. Overall, teenagers are performing better in tests but fewer get the very top marks, the researchers found. Opposition MPs said it was proof of slipping standards in schools, fuelling concerns that A-levels have got easier. A-level results have improved steadily over the same period.

Michael Shayer, professor of applied psychology at King's College London, tested 800 13- and 14-year-olds' ability to think analytically and logically. He applied a test identical to one carried out on the same number of pupils of the same age in 1976. In one of the tests, pupils were asked to study a pendulum swinging on a string and investigate what caused it to change speed. Some 24% of the teenagers gained high marks in this in 1976, while 11% did today. In another test, pupils were asked to think about what made weights balance on a beam. Some 20% gained the top marks in this a generation ago, while just 5% of today's teenagers managed this.

The brightest teenagers were far less clever than a generation ago, while their classmates of average intelligence are more able than they used to be, the research concluded. It found significant improvements in the average performance of pupils.

Shayer said teachers taught pupils basic skills because they "faced public pressure to improve scores for Sats", the national tests for 14-year-olds that were scrapped this month, but this was at the cost of stretching the cleverest children.

"Teachers are concentrating on giving the basic skills to more pupils, so the average ability goes up, but they fail to stretch the brightest, so the high-end ability falls," Shayer said. The findings echo concerns raised with the publication of this year's Sats results for 11-year-olds, which revealed a slight improvement in average results but a five percentage point dip in the proportion getting the top marks for English and slightly smaller falls in science and maths.

Shayer said his research showed children's responses were becoming quicker, but they lacked the ability to think "anything but shallowly".

"They are not as able to step back from reality and to reason," he said.

The King's College research, to be published in the British Journal of Educational Psychology, contradicts the year-on-year improvement in test results of 14-year-olds. It also fuels concerns that despite A-level grades rising for more than 20 years, exams are getting easier and standards are slipping.

The shadow education secretary, Michael Gove, said the study was a "powerful contribution" to the debate on academic standards.

"Government claims about rising standards are not justified," he added. "While the government tells us one thing about school improvement, the evidence points in another direction." He said the Conservatives wanted the brightest children to be picked out earlier so that they could be fully stretched at school.

Alan Smithers, director of education research at the University of Buckingham, said: "What we've tried to do is expand a good basic education to everyone with the national curriculum and tests at every stage in school. Those are reasonable steps, but once league tables and targets were introduced it put a financial incentive on schools to lift results overall, and the focus on the tests didn't help develop the highest achievers in the class."

He added that the introduction of new subjects, modular teaching and more directed questions had all made it easier to get the top marks at A-level. "A-levels have been made more accessible," he said.

John Dunford, general secretary of the Association of School and College Leaders, said ministers did not encourage schools to improve the performance of the brightest pupils by judging them on the number of pupils who achieved five A-C grades at GCSE.

He said: "Schools deserve praise for improving these thinking skills in the vast majority of pupils, and it is vital that they do this. However, the testing regime and the way schools are held accountable through the number of their pupils achieving good GCSE passes does not encourage schools to improve the performance of the brightest pupils. Schools will always try to provide bright children with adequate intellectual stimulation, but this is not how the government judges them."

A spokesman for the Department for Children, Schools and Families said: "Good teachers do not need to teach to the test and there is no evidence that such practice is widespread. We have already taken steps to reduce the testing burden, but targets and testing are integral features of any work to drive up standards."

Students' views

Sophie Wolfson, 15, a pupil at Highgate Wood school in north London

"It depends on the teachers as to whether the most able pupils are stretched at my school. Some give extra work, but others make you wait around if you have finished the work early. It would be good if there was a bit more for pupils who needed to be stretched. My school does help those who struggle. Pupils get mentors, but it doesn't affect the rest of us because the mentors just come in and we all get on with the work. Last year, when I was doing Sats, we were definitely working to the exams. That was all we did. It ruined English, maths and science because we weren't able to learn anything more than what might be in the tests. There was a lot of revision and memorising of facts. I don't remember much of what I learned. What we're doing for GCSEs is much more interesting. I haven't had any career advice yet. I'd appreciate some because I'm not sure what I want to do. I don't want someone to tell me what career I should do though."

Dylan, 14, from Islington, north London, attends a school in south-east London

"I don't feel stretched by my school. They think if you are smart that you know the stuff they are teaching already. If you are dumb, they put in more effort. The tests we did this summer at school didn't teach me anything. They just put us under pressure. The teachers know what level we're at without the tests. We just learned about the kind of questions we'd be asked. I found the tests hard. I didn't feel I revised enough. The careers advice I've had at school has been rubbish. They just made us fill out a form. If I'd had a few weeks, I would have been able to think about the answers, but I didn't have time. It doesn't matter though, I'll just ask other people for careers advice. I know I'm not going to university. I'm going to find a job maybe as an entrepreneur, salesman or property developer."